The invention relates to an output stage for an a.c. voltage switch, more particularly an a.c. voltage proximity switch, wherein a load is supplied from an a.c. voltage source via a rectifier which can be connected from an input stage via a control stage to a MOS-FET connected to the output of the rectifier, wherein the internal voltage supply of the input stage and the control stage, when the rectifier is not connected the input stage and the control stage are connected via a source or emitter follower with voltage-stabilized input to the a.c. voltage source, and when the rectifier is connected the input stage and the control stage are connected to a voltage-stabilizing electronic structural component at the output of the rectifier.
Output stages for a.c. voltage switches of the kind specified are normally operated with the mains a.c. voltage, the two output cables connected to the rectifier being connected to the mains via the useful load, for example, a relay, an indicator lamp or the like.
In one a.c. voltage switch used in practice, the rectifier can be connected by means of a thyristor triggered by the control stage. The thyristor is in series with a Zener diode at which the internal supply voltage is tapped when the rectifier is connected. The Zener diode connected to the anode of the thyristor stabilizes the supply voltage. When the rectifier is not connected, the internal voltage supply comes from the mains via a source follower connected to the output of the rectifier. An a.c. voltage switch of that kind has a number of disadvantages.
However, German Patent Specifications 31 46 709 C1 and 35 36 925 A1 also disclose a.c. voltage switches of the kind specified in which a MOS-FET is used instead of the thyristor.
For the currents of up to about 1 A involved in a.c. voltage switches, Zener diodes are obtainable only up to a minimum Z voltage of about 5 V. With an output current of the connected a.c. voltage switch of only 0.5 A flowing through the Zener diode, a power loss of about 2.5 W takes place. As a result, the a.c. voltage switch becomes progressively hotter, so that its switch points change. Moreover, Zener diodes are temperature-dependent with regard to their Z voltage, so that the connection and disconnection points of the a.c. voltage switch are displaced due to the internal supply voltage tapped at the Zener diode. Switch point stability further deteriorates due to the fact that the Z voltage depends on current. With an a.c. voltage switch output current of, for example 20 mA the Z voltage is about 20% lower than with a current of 0.5 A. Other disadvantages are the large amount of space required by the Zener diode and the fact that the heat is generated concentrated at a point, so that local damage may occur inside the switch. Lastly, the voltage drop in the connected-through condition along the series connection of a thyristor and a 5 V Zener diode is about 8 V; with a low mains voltage this value is too high for many applications, if a sufficiently high supply voltage is to be available.
In addition to these disadvantages resulting directly from the use of a Zener diode, another disadvantage of the a.c. voltage switch is that once a thyristor has been fired, it can be switched off again at the earliest only at the next crossover of the operating voltage. If during that time the output current exceeds the maximum permissible value, for example, due to the short circuiting of the external load, the thyristor is destroyed and the switch is no longer capable of operation. Effective protection against short circuiting cannot be provided at a reasonable cost.